1. Technical Field
The invention relates to GSM/GPRS cellular mobile station mobility, and more specifically to enhancements to cell change procedure during MBMS.
2. Discussion of Related Art
MBMS (Multimedia Broadcast/Multicast Service) is a unidirectional point to multipoint (p-t-m) bearer service in which data is transmitted from a single source entity to multiple recipients. These services are typically in the form of streaming video and audio.
Cell change of a mobile station during an MBMS session (with a serving base station/Node B of a radio access network of a wireless communication system) sometimes takes longer than is desirable, and of particular concern is when packet channels are supported by the target cell (i.e. the cell to which the mobile station is changing to for continued connection to the MBMS source). Due to the nature of the MBMS (in that it provides e.g. streaming video and audio) it is not desirable to have interruptions in the reception of data by the mobile station prior to or after a cell change. When the mobile station needs to perform cell reselection and the target cell supports packet channels, such as PBCCH (Packet Broadcast Control CHannel) or PCCCH (Packet Common Control CHannel), a GPRS attached mobile station camps on the packet channels. The PBCCH, if allocated, broadcasts packet data specific PSI (Packet System Information), but if PBCCH is not allocated, the packet data specific system information is broadcast on BCCH (Broadcast Control Channel. The PCCCH comprises logical channels, or subchannels, for GPRS common control signalling. These subchannels include: PRACH (Packet Random Access Channel), PPCH (Packet Paging Channel), PAGCH (Packet Access Grant Channel) and PNCH (Packet Notification Channel).
In order to camp on PCCCH the mobile station first needs to read PSI from PBCCH. In order to be able to read PSI the mobile station needs to know the location of the PBCCH, i.e. where it physically is located (in the radiofrequency spectrum), i.e. what its frequency is, and how it is scheduled. Currently a mobile station retrieves or receives the PBCCH description information either before or after a cell change. This can be done either by reading the information concerning the PBCCH allocation from a neighbor cell (prior to cell change) or by receiving the information on PBCCH from the serving cell.
A mobile station during data transfer may obtain the PBCCH allocation information prior to a cell change in at least three ways. A first way is for the mobile station to autonomously read the System Information (SI) of the target cell in order to receive the PBCCH description prior to the cell change. This is done independently of the data transfer, causing interrupts in the data transfer.
A second way is for the mobile station to apply Packet Cell Change Notification (CCN) if the network supports the Network Assisted Cell Change (NACC) procedure, and thereby receive the PBCCH description from the network prior to cell change. Note that NACC, and therefore also CCN, is currently only applicable in packet transfer mode, which does not include MBMS broadcast/multicast receive mode.
A third way is for the mobile station to receive the neighbor PBCCH allocation description in a PBCCH location information structure broadcast in Packet System Information (PSI) type 3 and type 3 bis, assuming the mobile station is camped on the packet channels. The PBCCH location information structure does not support broadcast of all types of PBCCH allocation descriptions; it only supports delivering the PBCCH allocation of a non-hopping PBCCH allocated on BCCH.
Non-hopping and hopping in respect to a channel have to do with the two forms of code division multiple access (CDMA) transmission, namely frequency hopping CDMA (FH-CDMA) and direct sequence CDMA (DS-CDMA). In an FH-CDMA system, a transmitter, in providing a communication channel, “hops” between available frequencies according to a specified algorithm, which can be either random or preplanned. The transmitter operates in synchronization with a receiver, which remains tuned to the same center frequency as the transmitter. A short burst of data is transmitted on a narrowband. Then, the transmitter tunes to another frequency and transmits again. The receiver thus is capable of hopping its frequency over a given bandwidth several times a second, transmitting on one frequency for a certain period of time, then hopping to another frequency and transmitting again. In DS-CDMA, data is chopped into small pieces and spread across a frequency domain.
If a mobile station attempting cell change does not retrieve or receive all of the PBCCH location information before the cell change, the mobile station first needs to camp on BCCH before camping on PBCCH. This is necessary in order to receive the PBCCH description information from System Information type 13 (SI13) broadcast on BCCH. This may take several seconds (up to approximately 7 seconds) due to scheduling rules concerning SI13. Afterward, the mobile station can switch to the PBCCH.
The main problem with retrieving the PBCCH description after a cell change is that the network needs to coordinate (if possible) the scheduling of the BCCH, the PBCCH, and the MBMS session. Such coordination is needed for the mobile station to be able to receive the BCCH, PBCCH and MBMS data simultaneously. As the mobile station cannot start to receive PBCCH data before having the PBCCH description, it is necessary also to ensure that the mobile station can receive BCCH data while receiving the MBMS data. This reduces the MBMS transmission capacity in the network because it reduces the maximum number of time slots possibly used for MBMS data transmission. More details are provided in 3GPP TS 43.246 MBMS Stage 2 specification Annex B. (Note that the above examples are used for illustrating the problem and may not cover all possible scenarios.)
Thus, the prior art currently does not make it possible to perform cell change during an MBMS reception to a cell supporting packet channels, without possible severe interruptions in receiving the MBMS transmission or by limiting the MBMS transmission capacity in cells supporting packet channels.
Therefore what is needed is a procedure for cell change that optimises and shortens the cell reselection time for a GPRS-capable mobile station engaged in an MBMS session, a procedure that is, ideally, of use in any GPRS cell change, and even to other wireless systems using cell reselection.